1. Field of the Invention
The present invention relates to torque limiting clutches and in particular to a temperature sensitive device for limiting the torque applied to the clutch.
2. Description of the Prior Art
The present invention is used in an upright vacuum sweeper which has a floor engagable brush dowel rotatably driven by a cog belt which engages a sprocket carried on the brush dowel. If the brush dowel becomes jammed such as by engagement with a sock or other article, a clutch mechanism permits the sprocket to slip on the brush dowel to prevent damage to the cog belt. The vacuum sweeper includes a control circuit such as that disclosed in U.S. Pat. No. 4,370,690 to terminate operation of the motor driving the brush dowel in the event the sensed speed of the brush drops below a predetermined low speed.
However, if the control circuit were to fail, and then the brush dowel is jammed, the motor will not turn off and the clutch will heat by friction reaching excessive temperatures. Since the brush dowel is oftentimes constructed of wood, the excessive heat will cause the wood to char and may in fact ignite.
Clutches having temperature sensitive torque limiting devices are disclosed for use in devices distinct from upright vacuum cleaners. For example, U.S. Pat. No. 4,287,975 discloses a clutch usable in an automobile which includes a meltable, fusible material between a friction lining material and its supporting structure in a drive plate and driven plate arrangement. The meltable, fusible material has a melting point such that it undergoes a transition from a solid state to a liquid state when the clutch reaches an elevated temperature, allowing the driven plate to slip with respect to the driving plate and thus preventing damage to the friction lining material. Upon cooling the assembly reforms to its original state.
U.S. Pat. No. 4,132,300 discloses the use of a solder connection between a drive shaft and an actuator assembly in a refrigerant compressor which will melt in the event the compressor becomes overheated. When the solder melts, a spring will cause the actuator assembly to move to cause keys to move out of engagement with a tapered section of a hub, releasing the driving connection and permitting free rotation of the pulley.